Engineering geologists and geotechnical engineers are an
intregal part of the design team for virtually all modern engineering
projects that involve site characterization and geotechnical design.
Evaluation of alternative project sites or specific site selection
usually requires data collection, analysis and explanation of
physical site conditions to other members of a project design team.
Because of the need to develop a mutual understanding of geologic
conditions and the resulting implications for design criteria, a
common understanding of the relationship between geologic origin and
geotechnical properties is essential. It is imperative that the
geologist and engineer work in close cooperation to assure the best
product quality.

Traditionally, the geologist's role has focused on
identification of the geologic origin and distribution of earth
materials. This includes both physical classification and
interpretation of the processes of emplacement and modification. The
product of a geologist's work within a project design team is often
primarily qualitative, usually a map with appropriate descriptions.
Such data must be translated into a quantitative form usable in
engineering analysis and in design development and evaluation. The
translation and quantification of geologic data for engineering
purposes occurs over a wide range of scales. Discussion of the
distribution of geologic materials and processes commonly involves a
megascopic scale of feet or miles, while many engineering properties
are discussed in microscopic context. A mutual understanding of
terms, units and properties is essential for geologists and engineers
to communicate effectively.

This paper relates the geologic characteristics and origin of
earth materials commonly found in Washington to certain geotechnical
properties. Four tables are presented in which descriptive and
interpretive properties of soil and rock materials are correlated
with their genetic classification.

The information presented in the tables is useful to indicate
the general range of values for typical geotechnical properties, but
is no substitute for site-specific laboratory and field information.
The tables will be of some direct benefit to students and to
geotechnical professionals who are new to the Pacific Northwest;
among those with local experience they will serve mainly as a basis
for ongoing argument.

The properties indicated in the tables are those most relevant
to geotechnical considerations. The values presented in the tables
are based on a compilation of published and unpublished information
and do not represent original research. These data have been compiled
from field and laboratory tests performed over many years by
engineers, geologists and geophysicists in both the government and
private sectors.

Because of the extremely variable nature of geologic
materials, the ranges presented in the tables should be considered
representative, but not necessarily all inclusive. Where ranges are
indicated, we estimate that roughly two-thirds of field or laboratory
observations will fall within the indicated ranges. Some geologic
categories are not described in the tables; for example, the tables
include no discussion of fill materials or landslide deposits because
it is the writers' opinion that these materials are too variable to
be meaningfully included. Not all pertinent geotechnical properties
are listed and some engineering projects will require information on
properties not included in the tables. The design team collectively
must evaluate what geological conditions might affect, or be affected
by, the engineering project.

DESCRIPTION OF TABLES

The four tables include summaries of descriptive and
interpretive properties of soil and rock. The vertical organization
of the tables is based on the genetic classification of the
materials; descriptive and interpretive properties of general
interest for engineering considerations are presented in the
horizontal headings. Unified Soil Classification System (USCS)
symbols are shown for soil materials and Unified Rock Classification
System (URCS) symbols are indicated for rock materials. These
classification systems are summarized in Figures 1 and 2. A
generalized explanation of terms is presented below, but is not
intended to rigorously define either the geologic categories or the
geotechnical properties.

o Cohesion: That part of the shear strength of soil or rock
which does not depend on interparticle friction.

o Permeability (Hydraulic Conductivity): The ease with which
water will move through soil interstices, expressed in feet per
minute. For rock, variability is so great that it is expressed in the
tables in dimensionless relative terms only. Negligible permeability
is expressed as 0.

o Storage Capacity (Specific Yield): The volume of water that
will drain from a unit volume of an unconfined aquifer.

o The Unified Soil Classification System (USCS) does not recognize
particles larger than 3 inches in diameter. Common usage extends
it to materials including cobbles (3 to 12 inches) and boulders
(greater than 12 inches).

o Cohesion is the result of soil structure and/or cementation. Some
finite cohesion is generally present in loess, due to its unique
granular structure and the common occurrence of minor cementation.
Cohesion in till is a result of ice consolidation and a wide range of
particle sizes, including a significant fraction of silt.

o Permeability differences reflect variations in gradation between
geologic materials. Very high permeability is associated with
high-energy alluvial deposits or glacial outwash where coarse, open-work
gravel is common. Permeability in these deposits can vary greatly
over short horizontal and vertical distances. Extremely low
permeability is associated with poorly to moderately sorted materials
that are ice-consolidated and contain a substantial fraction of silt
and clay.

o Storage capacity reflects the volume of void space and the content
of silt or clay within a soil deposit. Storage capacity is very small
for poorly sorted or ice-consolidated, fine-grained materials such as
till and glaciolacustrine deposits.

o Seismic velocities in soil can be affected by water content.
Coarse-grained soils display significantly higher velocities when
water saturated. Less velocity increase is associated with
finer-grained soils. The electrical resistivity of soil and rock
decreases with water content. Geophysical values are differentiated
between wet and dry conditions where differences are significant and
data is available.

Interpretive Properties

o Erodibility is closely related to slope, vegetative cover, water
concentration and numerous other factors in addition to geologic characteristics.

o Excavation difficulty is discussed in more detail in handbooks
published by Caterpillar, Inc. (1987a, b). Note that the table
entries for this category refer to unrestricted excavation.
Restricted excavations such as trenches are normally more difficult
than open cuts. Substantial variations from the indicated values
should be expected based on site-specific factors.

o Satisfactory foundation performance includes consideration of
numerous factors in addition to the indicated bearing values. These
factors include settlement performance, general stability and effects
of and on adjacent manmade or natural features.

o The design of safe cut slopes must consider site-specific details
of soil and water conditions and their relationship to risk. For
example, a maintenance risk is much less significant than a
life-threatening risk. Therefore, rather than relying on physical
properties, risk will often dictate slope design.

o Seismic hazards can be manifested in the form of ground shaking,
liquefaction, ground rupture or displacement (e.g., landslides
induced by seismic shaking). The extent to which the indicated
geologic classifications are associated with seismic hazards is
expressed in relative terms.

o Moisture sensitivity varies considerably within each geologic
classification. For example, low-energy alluvial deposits
characterized by clean, free-draining sand are not particularly
moisture-sensitive while low-energy alluvial soils containing a
substantial fraction of silt are extremely moisture-sensitive.
Although not included as a specific interpretive category for rock,
moisture sensitivity can also be important. The moisture sensitivity
of rock is generally proportional to the amount of clay or silt
produced by mechanical or chemical decomposition.

ACKNOWLEDGEMENTS

The writers wish to express appreciation to their colleagues
in the geotechnical professions who over the years have shared
information regarding geotechnical properties of geologic materials.
Several organizations (GeoEngineers, Inc., Geo-Recon International
Ltd., Shannon & Wilson, Inc., and the U.S. Army Corps of
Engineers) made available to us specific information from their
files. GeoEngineers, Inc., also provided assistance in manuscript preparation.

We are particularly grateful to Mr. George Yamane for his
helpful review and comments during the preparation of this paper.